Rolf Bruijn 1 , Luigi Burlini 1 & Karsten Kunze 2

1. Mechanical and microstructural development of Carrara marble during multi-stage deformation in torsion experiments Rolf Bruijn1, Luigi Burlini1 & Karsten Kunze2 1. Geological Institute ETH Zürich: Sonneggstrasse 5, CH-8092 Zürich, Switzerland 2. Electron Microscopy ETH Zürich: Wolfgang-Pauli-Strasse 16, CH-80931, Switzerland

2. Problem • Rock deformation experiments commonly restricted to single-phase deformation • Effect of strain on subsequent deformation hardly investigated • In nature: multiple deformation phases very common • Torsion experiments model high strain shear zone deformation • Natural shear zones commonly re-activate in continued and reversed sense of shear (e.g. basin inversion tectonics) Problem statement How does Carrara marble behave during multiple deformation phases, including continued and reversed shearing? & What are the consequences for the interpretation of natural shear zones in marbles? Problem 1/1

3. Method – Deformation phases Two deformation phases induced to sample by torsion 6 x D1: γ = 1, 2.6 OR 5, clockwise OR anti-clockwise 3x D2: γ = 1, 2.6 OR 5, anti-clockwise (simultaneous reversed & continued rotation) Note: gD1 = gD2 Schematic drawing to illustrate deformation history of Carrara marble samples used in this research Method 1/3

4. Method – Experimental procedure • Deformation apparatus = Paterson gas-medium HPT testing machine + torsion actuator • Deformation conditions: T = 1000 K, Pconf = 300 MPa, g-rate = 3x10-4 s-1. • Sample assembly D2 experiment: Assembly set-up, prior to jacketing Method 2/3

5. Method – Sample analysis Flow stress derived from measured torque data Flow stress is nominally equal for all three segments D2 strain estimates n = stress exponent (here n = 10, after e.g. Barnhoorn et al., 2004) M = torque (Nm) r = radius (here 7.5 mm) Thin section and EBSD sample cuts (modified from Pieri et al., 2001) Shear strain calculated from sheared iron jacket strain marker Method 3/3

6. Results: D2 strain variation • Strain markers after D2: strain variation between segments • Strain variation explained by different mechanical state • = 0.7 • g = 1.3 • g = 1.0 • = 4.1 • g = 0.9 • g = 2.4 • = 7.0 • g = 0.9 • g = 6.7 Strain variation after D2. Coloured lines indicate shear strain magnitude in relative order • No slip observed between segments or spacers/rods • Average shear strain = imposed shear strain Results 1/5

7. Results: D2 Flow behaviour Comparison between single-phase deformed and sandwiched Carrara marble Sandwich sample • Less and faster steady state • Steady state flow level decreases with increasing initial strain Explanation - Different mechanical states within sandwich sample - Fast deforming segments control flow stress evolution Shear strain/shear stress curves for D2 experiments and typical single-phase deformation flow stress evolution with strain Results 2/5

8. Results: Microstructures D2gbulk = 1.0 Top finite shear strain: gD1 + gD2 = 1.0 + 0.7 = 1.7 Centre finite shear strain: gD1 + gD2 = 0.0 + 1.3 = 1.3 Top finite shear strain: gD1 + gD2 = -1.0 + 1.1 = 0.1 Results 3/5

9. Results: Microstructures D2gbulk = 2.6 Top finite shear strain: gD1 + gD2 = 2.6 + 4.3 = 6.9 Centre finite shear strain: gD1 + gD2 = 0.0 + 0.9 = 0.9 Bottom finite shear strain: gD1 + gD2 = -2.6 + 2.6 = 0.0 Results 4/5

10. Results: Microstructures D2gbulk = 5.0 Top finite shear strain: gD1 + gD2 = 5.0 + 7.0 = 12.0 Centre finite shear strain: gD1 + gD2 = 0.0 + 0.9 = 0.9 Bottom finite shear strain: gD1 + gD2 = -5.0 + 6.7 = 1.7 Results 5/5

11. Conclusions Problem statement How does Carrara marble behave during multiple deformation phases, including continued and reversed shearing? & What are the consequences for the interpretation of natural shear zones in marbles? Conclusions 1/3

12. Conclusions Multi-phase deformation with continued rotation • Microstructure and texture very similar to those of single-phase deformation with similar finite strain • Fabric is determined by magnitude of finite strain • Flow is easiest when Carrara marble is already highly strained, followed by undeformed and hardest for weakly strained Carrara marble. Multi-phase deformation with reversed rotation • Low strains • Reversed rotation results in unique microstructure and relative strong CPO • Flow stress for reversed rotation is lower than for continued rotation (e.g. Bauschinger effect) • High strains • Fabric for reversed rotation is similar to high finite strain continued rotation fabric, despite large finite strain difference. • Flow stress for reversed rotation is slightly higher than for continued • Recrystallization fabric is determined by sum of absolute strains Conclusions 2/3

13. Conclusions Consequences • Estimating finite strain from a marble (ultra)mylonite fabric is hazardous, since no evidence for reversed shearing is recorded. Low strain sheared marbles - Softer in reversed sense of shear  strain localization mechanism - Harder than undeformed marbles High strain sheared marbles - Harder in reversed sense of shear - Softer than undeformed marbles  strain localization mechanism Conclusions 3/3

14. Thank you